1. Field of the Invention
This invention relates generally to a thrust bearing and more specifically to a thrust bearing having tilting pads supported by fluid equalized pistons.
2. Description of the Prior Art
While the construction of thrust bearings has become highly developed and very effective thrust bearings are now available for common use, bearings for high load situations are still somewhat imperfect. In shipboard main propulsion shaft systems, for example, conventional thrust bearings have not been satisfactory, due to several factors present in that environment. The loads on such shafts needed to drive the ship through the water are enormous, especially on larger ships, making the bearing load equally large. In addition, since the shaft must be sealed to prevent the introduction of seawater, very little tolerance is allowed for movement of the shaft. Misalignment and distortion of the shaft may also create problems, especially since the ship is operating in a rugged environment where heavy seas may inflict damage and maintenance facilities may not be available.
In order to combat these problems, specialized bearings have been designed for use in such environments. One such prior art device is shown in FIGS. 1 and 2. The thrust bearing, shown generally as 10 is formed in the shape of a ring having a central aperture 12 through which the shaft (not shown) passes. The shaft normally carries an integrally attached collar which contacts the thrust bearing. The ring is divided into a plurality of thrust pads (also known as thrust shoes) 14 separated by small spaces 16. The number of pads may vary, but there are commonly 8 or 10 in a ring. Each pad is supported from behind by a hydraulic piston 18 which is commonly connected to a pair of hydraulic oil manifolds 20. The manifolds 20 are connected to a common hydraulic line 22 which acts to supply additional oil to and receive excess oil from the manifolds 20. Position control valve 24 connects line 22 either to the oil supply or to the oil return depending on the needs of the manifold 20. Limit switches 26 are actuated by a dial indicator which senses the position of the piston 18. If the pistons 18 are either overextended or underextended, switches 26 close to supply power to solenoid 28 to control the valve 24 and the flow of oil. The manifold 20 may also be connected to accumulators so that the bearing manifold system acts to dampen out propeller vibrations.
FIG. 2 shows a cross-section of one of the pads. The pad 14 includes a Babbitt surface 30 and is carried on a pivot 32. The pivot 32 allows the pad 14 to move and produce a tapered wedge in the direction of shaft motion for fluid film hydrodynamic lubrication and also tilt in the radial direction to accommodate misalignment and distortions of the rotating thrust collar. The pivot 32 rests on piston 18 which slides in and out within a bushing 34 in response to thrust against the pad in front and hydraulic pressure from behind. Oil is supplied to cavity 35 from manifold 20 and pushes out against the piston 18. The bushing 34 is carried in a housing 36. When the thrust against one pad increases, the piston is forced back against the oil in the cavity increasing the pressure in the manifold. This causes an increase in pressure in the cavities of other pistons, thus forcing other pistons and pads outwardly to equalize the burden among the pads. Thus, uneveness in the shaft and its movement is handled by individual pads which can "wobble" on their pivots and by the equalizing effect of the manifold system. An indicator assembly 38 connected to one of the pistons indicates the movement of the piston in and out and controls the increase and decrease of oil in the system.
While this system has performed relatively well in ship shafting system environments, it is limited in the amount of thrust load it can handle, especially at low speeds. In addition, a certain amount of torque is required when starting the shaft to move.